1. Field of the Invention
The present invention relates to a body frame structure for a bicycle, and more particularly to a body frame structure suitable to augment the steering performance of the vehicle and support a high load applied to the body frame.
2. Description of the Background Art
Body frame structures for a bicycle, such as those described in (1) the official gazette of Japanese Patent Laid-open No. Hei 9-263282 “Bicycle Frame with a Buffer Apparatus” and (2) the official gazette of Japanese Patent Publication No. Hei 4-43834 “Frame for a Motorcycle,” have been available in the background art.
In FIG. 1 of document (1) mentioned hereinabove, a triangular frame is disclosed wherein an upper pipe 2 and a lower pipe 3 positioned on the lower side of the upper pipe 2 extend obliquely rearward and downward from a head pipe 1. An upright pipe 4 is connected to a rear end of the upper pipe 2 and a rear end of the lower pipe 3.
In FIG. 1 of the document (2) mentioned hereinabove, a motorcycle is described which includes a main frame member 5 extending obliquely rearward and downward from a head pipe 2, a rear arm 10 mounted at a rear end of the main frame member 5, and a hydraulic shock absorber 31 mounted on and between the main frame member 5 and the rear arm 10 through a link.
However, the present inventor has determined that the background art suffers from the following disadvantages. In the structure described in document (1), since each of the pieces of the triangular frame, which is formed from the upper pipe 2, lower pipe 3, and upright pipe 4, have a fixed outside dimension from one end to the other end thereof, the rigidity is substantially fixed at any portion of the pipe.
As the rigidity of a body frame increases, the distortion or twist of the body frame decreases. Therefore, this is advantageous to a high load transmitted thereto from the road surface and also the sensitivity of the body frame to increases in load. However, if the rigidity of the body frame is excessively high, e.g., when the body frame is acted upon by an impact load from the load surface, the body frame reacts quickly to the impact load. Therefore, the driver is liable to get tired because the driver is forced to react with the movement of the body. Further, since the body frame is less likely to undergo twisting, a good turning characteristic is not obtained.
In contrast, if the rigidity of the body frame is excessively low, then an excessively great twist of the body may occur or precise road surface information is not likely to be conveyed to the driver. Accordingly, since the body frame reacts poorly to the movement of the driver, it is difficult to undertake quick motion.
For example, in a downhill race where bicycles run on considerably rough roads along corners at high speed, it is desirable for the body frame to include a portion having a high rigidity and another portion having a low rigidity for the reasons described hereinabove. With respect to the triangular frame of the document (1) described hereinabove, it is difficult to satisfy both the supporting of a high load and the augmentation of the steering performance.
Meanwhile, with the structure described hereinabove in document (2), if the rear arm 10 swings upward and downward, then a bending load acts upon an intermediate portion of the main frame member 5 from the rear arm 10 through the link and the hydraulic shock absorber 31. Accordingly, the rigidity of the main frame member 5 must be increased to adequately withstand the bending load. However, if the rigidity of the main frame member 5 is increased, then the weight of the body is increased and the performance of the bicycle is deteriorated.